Metal components in the working gas path in a gas turbine engine may have a ceramic thermal barrier coating (TBC). Random voids can form in the TBC from sintering shrinkage during processing. These voids can be beneficial for thermal insulation, but detrimental to durability of the TBC and the metal substrate. The amount and size of voids formed and the resulting porosity are not controlled variables. High temperatures in the gas turbine environment can cause changes in the properties of the ceramic TBC, including further sintering, that can lead to spalling of the TBC. Additionally, the thickness of a ceramic TBC is limited due to the mismatch of processing shrinkage between the ceramic coating and the metallic substrate and from differential thermal expansion during operation cycles. This thinness limits the amount of oxidation protection, abrasion protection, and insulation that can be provided by a TBC, and limits its life.
A common method of joining of ceramic coatings to metal substrates is by surface deposition techniques such as High Velocity Oxygen Fuel spraying (HVOF), Air Plasma Spraying (APS), and Physical Vapor Deposition (PVD). However, these methods provide a non-chemical bond with limited durability.
Ceramic powders and metal powders can be formed into desired shapes, and then sintered to form dense bodies that can be structural. Such fabrication offers rapid manufacturing of net-shape parts. However, the sintering shrinkage of typical metal powders is about 6%, while that of typical ceramic powders is about 1%, and ceramics require much higher sintering temperatures than metals. These sintering disparities, plus different thermal expansion rates and different mechanisms of bonding between atoms, make a stress free bonding of ceramics to metals very difficult under normal circumstances. Such bonding is needed for ceramic coatings on metal components of gas turbine engines, where the ceramic coating serves as a thermal barrier and/or provides object impact resistance and/or desirable abrasion characteristics. Some gas turbine components are exposed to temperatures that cycle from ambient temperatures to about 1,500° C. between shut-down and operational phases of the gas turbine. Operation at such high temperatures causes continued densification of the ceramic material over time.